Could Cedric have been using Cho to help him answer the riddles? And did Krum use Hermione?

no, if you remember, ron accused hermione that krum only wanted help figuring out the clue, and hermione said that they had never discussed the clue and that she wants harry to win. with regard to fleur and roger davies, well the madame maxime's lot always hung out at the ravenclaw table, and fleur, with the kind of person she is, would pick the most popular ravenclaw boy to take to the dance, obviously the quidditch captain. with regard to cho and cedric, well cho, harry states, is very pretty and popular. cedric MAY have asked her just for help with the clue, but it is also possible he genuinely liked her. hope that helped!